1. Technical Field
This invention relates to an apparatus for the surveillance of an electronic security element in an interrogation zone, more specifically to such an apparatus which has a transmitting device emitting at least one cyclic interrogation signal into the interrogation zone, with the interrogation signal causing the security element to deliver a characteristic signal, a receiving device which receives the characteristic signal, and a computing/control unit which evaluates the signals received from the receiving device and produces an alarm when the presence of the security element is established.
2. Background Art
To detect the presence of electromagnetic security elements in an interrogation zone, it is proposed in European patent, EP 123 586 B, to emit into the interrogation zone, in addition to two interrogation fields with the frequencies F1 and F2 in the kilohertz range, a field with a frequency F3 in the hertz range. The two interrogation fields with the frequencies F1 and F2 cause a security element present in the interrogation zone to emit a characteristic signal with the intermodulation frequencies n.multidot.F1.+-.m.multidot.F2 (where n, m=0, 1, 2, . . . ). The low-frequency interrogation field causes the security element to be driven from saturation in one direction into saturation in the other direction at the clock rate of this particular field. As a result, the characteristic signal occurs cyclically at the frequency of the low-frequency field.
As an alternative solution, it has further become known to use only one interrogation field in the kilohertz range for excitation of the security element, with the characteristic signal of the security element occurring again at the clock rate of a low-frequency field cycling the magnetically soft, non-linear material between the two states of saturation.
Further, it has been known in the art to remove from the received signals interferences bearing a relation to the known frequency of the power supply or its harmonics, enabling the measuring accuracy of the surveillance apparatus to be materially increased. A suitable method is described, for example, in European patent EP 0 431 341 B1.
It is a disadvantage of hitherto known surveillance apparatus that they are only suitable for the elimination of interference signals with known frequencies. They fail in the presence of temporary cyclic interference signals which bear no relation to the frequency of the power supply. Such interference signals are, for example, produced by a switching power supply for a cash register.